I've been using Redline 75W 90 NS in my transmission and Redline 75W 90 in my rear differential since 3,000 miles and now I'm getting close to 30,000, so I'm gonna be changing it again pretty soon. Part of me wants to be brave and try Uncle Scotty's Cocktail in the trans, but I haven't had any problems with my current setup, so I'm probably gonna stick with that. I also like knowing that if I ever have to add any fluid, I won't have to measure out a certain ratio of several different kinds of oil like I'd probably have to if I used Uncle Scotty's Cocktail. I'm a believer that as long as it's synthetic, it should perform slightly better than conventional. Unless someone has done a specific test and can cite facts that one type or combination of oils significantly outperforms the rest, it's mostly personal preference.